Use system python libraries rather than virtualenv

[danvk]

Something I've noticed over a few happy years of using git-helpers: if I'm in a virtualenv and run gln, it fails unless I also run pip install python-dateutil inside the virtualenv.

I'm not sure what the standard practice is for distributing Python programs, but it feels like installing python-dateutil once globally should be sufficient. Maybe gln should run deactivate?

[ryan-williams]

Interesting, yea idk what the right way to handle that is! Open to what you think